Heterodyne receivers are one type of receivers used for RF signal down-conversion. Heterodyne receivers work by down-converting the RF signal into an intermediate frequency (IF) signal, filtering the IF signal to remove any interfering signals, and amplifying the filtered signal before another step of down-conversion to a baseband frequency.
Another promising RF down-conversion architecture, direct-conversion, eliminates the conversion-to-IF step, and directly down-converts the RF signal to baseband frequency. Without the IF stage, several elements of a wireless receiver can be eliminated effectively reducing its size and cost.
Despite its size and cost advantages however, direct-conversion inherently suffers from a “self-mixing” problem. As an undesirable effect of the local oscillator signal mixing with the received RF signal, self-mixing results in a DC offset being added to the down-converted signal which may saturate circuit elements in following stages of the receiver depending on the applied gain in these stages. Another source of DC offset, since gain is applied after down-conversion, is due to the gain stages introducing residual offsets due imperfections.
While, typically, a DC offset can be easily removed using a high-pass filter circuit with an appropriately set roll-off frequency, the problem is more challenging in the case of a wireless receiver circuit. In a wireless receiver, gain control is needed due to the varying nature of received signal levels. It desirable for several reasons, among which is reducing the area of the receiver, to implement a mixed gain and high-pass filtering architecture as opposed to having separate cascaded gain and high-pass filtering elements. In this architecture, as a result, the roll-off frequency of a wireless receiver circuit changes constantly with changes in the gain of the circuit. A tradeoff therefore exists between gain control and DC offset cancellation in the wireless receiver.
What is needed therefore is a wireless receiver circuit with independently configurable gain and roll-off frequency. Further, a method for varying the gain and the roll-off frequency of the receiver independently of each other is also needed.